Electrically small cavity antenna

ABSTRACT

An electrically small cavity antenna especially adapted for flush mounting in an aircraft vertical stabilizer. A cavity is formed in the vertical stabilizer so as to have a pair of radiation windows or apertures on opposite sides of the stabilizer. An energy coupling device is placed substantially in the center of the cavity to couple energy from a feeding network to the cavity. The energy coupling device includes a pair of spaced-apart plates with a tapered portion disposed between the plates. Energy is coupled from the feed network to one of the plates and the narrow end or apex of the tapered portion.

United States Patent [72) Inventor John Joseph Blasko Nashua, NH. [21]App]. No. 823,566 [22] Filed May 12, 1969 [45] Patented Oct. 12, 1971[73] Assignee Sanders Associates, Inc.

Nashua, NH.

[54] ELECTRICALLY SMALL CAVITY ANTENNA 4 Claims, 4 Drawing Figs.

[52] US. Cl 343/708, 343/769, 343/789 [51] Int. Cl H0lq 1/28 [50] Fieldof Search 343/705, 708, 773, 774, 775, 789, 830, 908, 769

[56] References Cited UNITED STATES PATENTS 2,644,090 6/1953 Dome343/789 Primary ExaminerEli Lieberman Attorney-Louis Etlinger ABSTRACT:An electrically small cavity antenna especially adapted for flushmounting in an aircraft vertical stabilizer. A cavity is formed in thevertical stabilizer so as to have a pair of radiation windows orapertures on opposite sides of the stabilizer. An energy coupling deviceis placed substantially in the center of the cavity to couple energyfrom a feeding network to the cavity. The energy coupling deviceincludes a pair of spaced-apart plates with a tapered portion disposedbetween the plates. Energy is coupled from the feed network to one ofthe plates and the narrow end or apex of the tapered portion.

PATENTEDnm 12 ml 3.6 1 3 .098

sum 1 c? 2 lNVli'NTUR. JOHN JOSEPH BLASKO ATTORNEY PATENTEDUEI 12 IQYI3.6 1 3 .098 SHEET 2 [1F 2 FIG?) IN VEN'H m. JOHN JOSEPH BLASKO ATTORNEYELECTRICALLY SMALL CAVITY ANTENNA BACKGROUND OF THE INVENTION Thisinvention relates to a new and improved antenna and, in particular, to acavity type antenna which is of relatively small physical size.

The fundamental limitations imposed on the performance of a small cavityantenna by its physical size are well known. In general, as a cavityantenna is made smaller, its radiation efficiency and bandwidth becomesmaller. Such limitations, as the foregoing, become most severe when themaximum physical dimension of the cavity antenna aperture isconsiderably less than one-half of a free space wavelength at the lowestfrequency of interest.

For such small cavity antennas, the bandwidth can be increased byresistive loading. However, such resistive loading decreases theradiation efficiency such that the gain is reduced by the same factor asthe bandwidth is increased. That is, the overall gain is the product ofthe directive gain and the radiation efficiency.

For example, at the very high frequency (VHF) range of communicationsused by aviation (118 to 136 megahertz, Mhz), a free space halfwavelength corresponds to an aperture dimension on the order of 4 feet.It is apparent that, for most aircraft, a cavity having aperturedimensions on the order of 4 feet would seriously disrupt the design ofthe aircraft for either external or internal (flush) mounting. Byemploying the aforementioned resistive loading technique, low efficiencycavity antenna dimensions on the order of 20 inches have been flushmounted in relatively large aircraft in the navigation band (108 Mhz.).However, such resistively loaded cavity antennas not only have lowefficiency, but are impractical for use in smaller aircraft.

BRIEF SUMMARY OF THE INVENTION An object of this invention is to providea new and improved cavity antenna.

Another object is to provide a novel cavity antenna having aperturedimensions considerably less than one-half wavelength without resistiveloading but yet having the electrical characteristics of a much largercavity antenna.

Yet another object is to provide a new and improved energy couplingdevice.

In brief, a cavity antenna embodying the present invention includes acavity having at least one aperture and an energy coupling devicesupportably mounted within the cavity. The coupling device includes anelectrically conductive tapered portion having a narrow and a wide end.An electrically conductive plate is supported adjacent the narrow end ofthe tapered portion. Energy feeding means has separate connections tothe plate and to the narrow end of the tapered portion.

In one embodiment of the invention, the tapered portion has the shape ofthe cone while in another embodiment it has a multisided shape. Ineither case, another electrically conductive plate is preferablyattached to and covers the wider end of the cone or multisided taperedportion.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, like referencecharacters denote like structural elements; and

FIG. 1 is a dimensional view of a flush mounted cavity antenna embodyingthe invention; and

FIG. 2 is a planned view of an energy coupling device embodying thepresent invention; and

FIG. 3 is a sectional view taken along the lines of 3--3 of FIG. 2; and

FIG. 4 is a dimensional view of another energy coupling device embodyingthe invention.

DESCRIPTION OF PREFERRED EMBODIMENT Although the cavity antenna of thepresent invention can be used in any desired frequency range, it isespecially useful for VHF communications. By way of example andcompleteness of description, the cavity antenna of the invention, hereinillustrated, is embodied as a flush mounted aircraft communica tionsantenna. Generally, VHF communications aviation band antennas arerequired to operate over the frequency range of l 18 to I36 Megahertz(Mhz.), to have a voltage standing wave ratio (VSWR) of less than 2:1,to have a nominal gain of zero decibel (db.), to have verticalpolarization and to have an essentially omnidirectional radiationpattern in the azimuth plane.

In the flush mounted embodiment shown in FIG. 1, a cavity 10 is formedin a suitable portion of the aircraft frame, such as vertical stabilizerl 1, of which only a portion is illustrated. The cavity 10 is formed ina suitable portion of the aircraft frame, such as vertical stabilizerll, of which only a portion is illustrated. The cavity 10 has a pair ofsidewalls l2 and 13, a top wall 14, and a bottom wall 15; each wallbeing of a suitable electrically conductive material, such as metal, forexample, aluminum. Each of the walls may have a geometrical contour, asillustrated, so as to conform to the contour and structural design ofthe aircraft. The cavity 10 also has a pair of apertures (or windows)disposed on opposite sides of vertical stabilizer l1 and having suitableradomes 16 and 17 attached thereto. The radomes I6 and 17 may be of anysuitable electromagnetically transparent material, such as fiberglass,and are attached to stabilizer 11 by any suitable fastening means, suchas screws or bolts (not shown).

Radome 17 is shown in FIG. 1 as removed from stabilizer 11 in order toconveniently illustrate the cavity 10 and an energy coupling device 18,which is disposed within cavity 10 when assembled. The arrows in FIG. 1illustrate the respective locations of coupling device 18 and radome 17relative to cavity 10 when in assembled form.

The coupling device 18 includes front and backplate portions 19 and 20,respectively, and a tapered or flared portion 21 disposed therebetween.The plates 19 and 20, and the tapered portion 21 may be any suitableelectrically conductive material; such as metal, for example, brass,aluminum, copper, and the like. The tapered portion 21, which maysuitably have the shape of a cone, has its wide end attached to frontplate 19 by any suitable means, for example, a solder or weld joint. Thecone 2] is secured by way of a dielectric support 22 to backplate 20such that the narrow end (apex) of the cone is electrically isolated, asby spacing, from backplate 20 (best seen in FIG. 3). The dielectricsupport 22 is attached to the backplate 20 and to the cone 21 by anysuitable means, such as bonding or hardware. The backplate 20 has beenpartially broken in FIG. 1 in order to illustrate the support 22.

The coupling device 18 is fed by any suitable feeding device 23, such asthe illustrated coaxial cable. The cable 23 extends downwardly throughthe bottom wall 15 of cavity 10 into the vertical stabilizer II andhence to the aircraft fuselage (not shown) where it is connected to atransmitting and/or receiving device (also not shown). The attachment ofcable 23 to the coupling device 18 is best seen in FIGS. 2 and 3. Thecentral conductor 24 is attached, as by soldering, to the narrow end ofthe cone 2] and the outer conductor 25 is attached to the backplate 20.Although the connecting order of the central and outer conductors may bereversed, the illustrated order is preferred for lightening protection.In assembled form, the coupling device 18 is located substantially inthe center of cavity 10 and is supported therein by any suitablesupporting means, preferably of a low dielectric material. For example,in one cavity antenna embodiment actually constructed, the couplingdevice 18 has been supported in cavity 10 by means of a low dielectricconstant foamlike material. Also in the assembled form, the radomecovering 17 is fastened to the vertical stabilizer 11 so as to fit overthe cavity aperture and secure the supporting foam and coupling device18 within the cavity.

For impedance matching purposes, a stub 26 (for example, a strip ofmetal) is coupled between he backplate 20 of coupling device 18 and thetop wall 14 of cavity 10. The stub 26 may be fastened to backplate 20and the cavity wall 14 by any suitable electrical connecting techniqueas, for example, a solder joint or a bolted joint. The width and thelength of stub 26 are preferably selected for optimum impedance matchingof cavity with coupling device 18 and feed cable 23. Although stub 26may have other locations (e.g., between cone 21 and plate 20), itpreferably has the illustrated location for mechanical support as wellas lightening protection.

The coupling device 18 serves the function of transferring energybetween cavity 10 and the feed cable 23. It has been found that thecoupling device 18 permits the apertures of cavity 10 to have physicaldimensions substantially smaller than one-half of a free spacewavelength at the lowest frequency of interest, but yet provide theelectrical characteristics of a much larger cavity. For example, in onecavity antenna design for the VHF aviation communication band, theaperture dimensions were approximately 12 inches. That is, the cavitywalls 12, 13, 14 and 15 were each approximately 12 inches in lengthwhich corresponds to an aperture dimension on the order of one-tenth ofa free space wavelength at the lowest frequency (1 l8 Mhz.) of interest.The azimuth patterns of the constructed cavity antenna (which wasvertically polarized) were omnidirectional within 1 db. and weremeasured every 2 Mhz. from 118 to 136 Mhz. The gains, which weremeasured by comparing the antenna to a vertically polarized dipole,varied from 2.0 to 1.0 db. with respect to the dipole. The antenna rangeused to measure the radiation patterns and relative gain was a IOO-footground plane range calibrated for measurements in the VHF band. The VSWRwas less than 2:1 throughout the frequency band of 118 to 136 Mhz.

The conical shape of the tapered portion 21 is preferred since it can beeasily fabricated at low cost. However, other tapered shapes can beemployed. For example, a multisided shape, such as the four sided(pyramidal) shape 27 illustrated in FIG. 4 can be used. The plate 19 ofthe conical and pyramidal coupling device embodiments is desirable toprovide symmetry as well as to provide an area which is compatible withthe area of the aperture. However, it is to be noted that the cavityantenna will also operate with plate 19 removed.

It should also be noted that cavity 10 could be located in portions ofthe airframe other than the vertical stabilizer l1 and that the antennacan be designed for frequency bands other than the aviation band.Indeed, the cavity antenna can be embodied in applications other thanflush mounted aircraft applications.

What is claimed is:

1. A cavity antenna comprising a cavity enclosure having first andsecond apertures on opposite sides thereof;

an energy coupling device including first and second electricallyconductive plates an electrically conductive tapered portion having anarrow end supported adjacent the first plate and a wide end which isattached to and covered by the second plate;

means for supportably mounting said coupling device substantially inthe'center of the cavity with the first and second plates facing thefirst and second cavity apertures, respectively; and

energy feeding means having separate connections to the first plate andto the narrow end of the tapered portion.

2. The invention according to claim 1 wherein said tapered portion has aconical outer surface,

said narrow end corresponding to the apex of said conical surface.

3. The invention according to claim 1 wherein said tapered portion has amultisided outer surface,

said narrow end corresponding to an apex of said multisided surface.

4. The invention according to claim 2 wherein said cavity is formed inthe vertical stabilizer of an aircraft, the cavity apertures beinglocated on opposite sides of said stabilizer such that the cavityextends between the apertures and through said stabilizer.

1. A cavity antenna comprising a cavity enclosure having first anDsecond apertures on opposite sides thereof; an energy coupling deviceincluding first and second electrically conductive plates anelectrically conductive tapered portion having a narrow end supportedadjacent the first plate and a wide end which is attached to and coveredby the second plate; means for supportably mounting said coupling devicesubstantially in the center of the cavity with the first and secondplates facing the first and second cavity apertures, respectively; andenergy feeding means having separate connections to the first plate andto the narrow end of the tapered portion.
 2. The invention according toclaim 1 wherein said tapered portion has a conical outer surface, saidnarrow end corresponding to the apex of said conical surface.
 3. Theinvention according to claim 1 wherein said tapered portion has amultisided outer surface, said narrow end corresponding to an apex ofsaid multisided surface.
 4. The invention according to claim 2 whereinsaid cavity is formed in the vertical stabilizer of an aircraft, thecavity apertures being located on opposite sides of said stabilizer suchthat the cavity extends between the apertures and through saidstabilizer.